A one system law allows for the law of the country to be evenly applied. Women are especially vunerable to two system laws such as have parallel systems of sharia or beth din being recognized on the same level the one system law such as UK. Sharia and Beth din courts are put into place by religious organizations that have a detrimental impact upon women and children because of their patriarchal nature. Many times women are forced into these religious courts because of family or the husband resulting in complete loss of children, child support, inheritance and the list goes on. In addition, many times it is quite easy for the husband to obtain a divorce on his word along but a wife must jump through hoops or pay extortion money. Often these proceedings are not recorded and the women have no attorney to represent them. They must sign documentation that states that they will abide by the ruling. The courts rarely have any female involvement or female insight. There have been cases where husbands refuse to give a "get" for a jewish woman resulting in her being a chained woman for years while the husband can remarry through loopholes found by the rabbi. This happens a lot with muslim husbands also they abandon their wives and often won't grant the divorce. Many communities even in the US (Beth din) will ostrasize women but accept the man even if the man has committed great abuse to the woman and/or children. Beth din is currently allowed in the US but not Sharia courts. This system creates division among those who insist upon their religion to be considered in court. Take for insistance that muslim imam state that women cannot be raped by their husbands or that the koran allows for women to be beat. So if you are a muslim women then you must accept this ruling but those who are not don't. I am not aware of any courts set up by Christians at this time but they used to have the canon laws. We can certainly see how the Roman Catholic church treats crimes by their bishops regarding sex crimes on children they hide it. They are also notariously patriarchal as well. Take the incident I believe in Brazil where a step-father raped his 9 year old daughter. She became pregnant with twins and the doctors felt all would die if she went forward with the pregnancy. With the consent of the mother the child had an abortion. The church ex-communicated everyone involved in the abortion except the step-father. The Roman Catholic Church certainly has made it's statement that rape is not a crime both with it's Bishops conduct but also with the incident that occurred in 2009. Now think about having them set up a court to assist people especially women and children. Do you think they will ever see any justice, I don't think so. I am a great proponent however for getting rid of the beth din courts currently in the United States as this is currently a two-system law currenly operating here. I think the US needs to implement laws that forbid religious marriages that don't register with the State as well as having a criminal sentencing for the the rabbi, priest or imam who doesn't register it. This would assist in eliminating polygomy in the US a well.
A one-system law is best. The legal system in the United States today is complex enough, so there is no need to further complicate things by adding more.
A two-system law, on its face, is a rejection of freedom and equality in a society, since it presupposes two tiers of the rule of law for separate segments of that society. For a nation to function as a whole, all citizens and all classes must be subject to the same set of laws.
As a nation, the people should show some type of unity, no matter their differences. In certain situations, they should act together as a whole and feel equal within the country. A two-system law hinders this unity and equality. Therefore, there should not be a two-system law, but rather only one system.